An engine produces energy by combusting a fuel. A gas turbine engine is a type of engine that produces energy by combusting a mixture of fuel and air. A gaseous fuel used in gas turbine engines is natural gas. Natural gas may be transported from a location where it is produced to a location where it is consumed by tanker ships equipped with cryogenic compartments. Such a tanker may be referred to as an LNG carrier. Since liquefied natural gas (“LNG”) takes up only a fraction (about 1/600) of the volume of natural gas in its gaseous state, natural gas is transported in the LNG carrier in a liquefied state. LNG may be produced by cooling natural gas below its boiling point (about −259° F. at ambient pressure). LNG may be stored in cryogenic containers either at or slightly above atmospheric pressure in the LNG carrier. By raising the temperature of the LNG, it may be converted back to its gaseous form. LNG carriers transport LNG over regions with widely varying temperatures. For instance, natural gas produced in locations such as Algeria, Borneo, or Indonesia, may be liquefied and transported on LNG carriers to Europe, Japan, or the United States. During the course of a voyage, the LNG carried as cargo heats up and boils off at an average rate of approximately 0.12% to 0.15% of the cargo volume per day. This portion of the LNG cargo that boils off is termed BOG (Boil off Gas). In this disclosure, liquefied fuel is generally referred to as LNG.
LNG carriers have traditionally been propelled by steam turbines and diesel engines. These propulsion systems use the BOG to assist in generating power to propel the carrier. For instance, steam turbine propulsion systems use BOG to boil water to produce the necessary steam, and dual fuel diesel engines use BOG in addition to diesel as fuel for the engine. Steam turbines and diesel engines are relatively heavy and bulky, and therefore, may decease the amount of LNG that may be transported using the carrier. Gas turbine engines (GTE), which are more compact and offer a significantly higher power-to-weight ratio than diesel engines and steam turbines, have also been used to propel LNG carriers. These GTE's use BOG as a portion of the fuel needed to operate the GTE.
In a GTE, a mixture of compressed air and fuel is ignited in a combustor to produce high pressure and high temperature combustion gases. The resulting hot gases are directed over the turbine's blades, spinning the turbine, thereby, producing mechanical power. This mechanical power may be used to operate other systems of the LNG carrier. For instance, the mechanical power may be directly used to turn a propeller of the carrier, or may be used to produce electric power which, in turn, drives an electric motor coupled to the propeller. There are a number of constituents of the combustion gases that are regulated by governments of countries through which an LNG carrier operates. One of these constituents is NOx. NOx is produced when nitrogen and oxygen, that are present in combustion gases, are subject to high combustion temperatures. NOx emissions from gas turbines may be lower than from diesel engines. NOx emissions and power output of gas turbines varies as a function of the intake air temperature. When ambient air temperature increases, air density decreases, and power output from the GTE decreases. With decreasing air density, NOx emissions from the GTE also decrease, until additional fuel may be required to maintain the power output. At that point, NOx emissions increase. Therefore, temperature of the air ingested by the GTE may be controlled to maintain NOx emissions within acceptable limits while increasing power production. The thermal efficiency of a gas turbine, at a defined load, varies with the intake air temperature. In the same way as described before, the temperature of the air ingested can also be controlled to optimize the thermal efficiency of the gas turbine for a given load. Another constituent of combustion gases that may be regulated is CO2. For a given load and a given fuel, the emissions of CO2 depend only in the thermal efficiency of the gas turbine. Optimizing the efficiency of a gas turbine therefore allows lower fuel consumption and a reduction of CO2 for a given task.
U.S. Pat. No. 3,387,462 issued to Bauger et al. (the '462 patent) discloses a gas turbine engine that propels a ship carrying a cargo of liquefied combustible gas. A combination of a conventional fuel and vaporized combustible gas is used to power the gas turbine of the '462 patent. The conventional fuel is directed to the combustion chamber from the ships conventional fuel tanks, while the vaporized combustible gas is directed into the combustion chamber from the ships cargo tanks. While the '462 patent discloses a GTE for a liquefied fuel carrier application that partly uses the vaporized fuel for operation, the '462 patent does not modulate power production and NOx emissions from the GTE by ingested air temperature control.